Q:
I am intending to add a bean probe to my roaster and I am
confused by the different probes, and where to place the probe to most
accurately read bean temperature: can you help?
A:
The widespread use of temperature probes in coffee roasters
over the last decade or so, even by “old school” or “artisan” roasters has
helped make our industry more professional and safer. Additionally, it has helped spark the
movement toward the profiling of coffee by more accurately, and more quickly,
measuring changes along the roast time and temperature curve. In coffee roasting we are generally
discussing 3 types of probes: J type thermocouples, K type thermocouples, and
RTDs. Let’s start with the
thermocouples.
Thermocouples (Type
Js and Ks)
All thermocouples measure temperature in the same way: by
utilizing the known temperature gradient of dissimilar metals. These dissimilar metal wires are housed
within the outer sheath of the probe, most often made of stainless steel. The heating and cooling of these metals
create EMF (electromotive force) that is read as voltage this is then
translated into a temperature and read by a repeater, controller or
computer. Both J and K thermocouples
work in this manner. The practical difference between a J and a K is the range
and therefore the accuracy of each type.
A J type thermocouple can measure up to 1380 F, while a K can measure up
to 2300 F. This means that Js are more
temperature sensitive than Ks. Because
coffee seldom sees temperatures much above 470F the more discreet readings of
the J are a better fit for use as a bean probe.
So, why use type K thermocouples at all. Type K thermocouples have their use in coffee
roasting operations for the reading of temperatures in afterburners. The higher temperatures required when
operating afterburners are at the very limit of what a J can read and continued
operation at this level will eventually cause a J probe failure, making Ks much
more practical for this operation. Also,
the more discreet readings required in the coffee roasting process are not
needed in the operation of pollution control devices.
RTDs (Resistive
Temperature Device)
Resistive temperature devices (RTDs) measure temperature
differently from thermocouples: RTDs work by measuring temperature induced
electrical resistance across the elements.
Once again the elements are housed in a stainless sheath similar to the
thermocouples.
Type J versus RTD
While either a J type thermocouple or an RTD are well suited
for use as a bean probe there are definite and distinct differences between the
two. Below is a quick reference list of
differences:
RTD
- Able to accurately and easily calibrate
- Easier to bend without damaging
- More accurate
- More stable throughout the profile
J Type Thermocouples
- 2-3 times cheaper than RTDs
- More durable
- Responds faster
The increased accuracy of the RTD comes from the linear
nature of the temperature vs. resistance plot, as well as their better
stability. Thermocouples on the other
hand are decidedly non-linear leading to more drift from profile to profile, this
temperature drift decreases stability.
Generally speaking, most roasters using either a basic set point
controller (or using a PID controller in this manner), or a repeater are fine
with a J type thermocouple. While many,
if not most profile control systems (and better profiling data-loggers) are
using RTDs as these systems are more able to capitalize on the increases in
accuracy and stability, thereby justifying the increased cost.
At the end of the day, the decision ultimately lies with the
owner/operator, as both of these probe types will work well- so long as
whatever is being used to read the temperature can accept the signal from the chosen
probe type. And, they are placed so that they can measure the surface
temperature of the coffee.
Placement for
accuracy
At the risk of sounding too obvious, if you want a probe to
read bean temperature, then you need to place it where it is in contact with
the beans. In fact, the probe needs to
be immersed in the coffee. For front
opening drum roasters this can be a pretty simple part of the process (see
drawings). For certain types of drum
roasters, those where the coffee does not ride up against the faceplate; side
openers, bottom openers, etc… then some modification maybe needed in order to
accurately measure bean temperature, this modification is often accomplished
with a funnel shaped catch that allows a mass of beans to surround the
probe. Fluid bed roasters present a
special challenge to the measurement of bean temperature as well. The larger volume of air, and the movement of
the coffee within the chamber, while consistent, will read higher temps than
drum roasters; making it difficult for fluid bed and drum roaster operators to
exchange accurate roasting information.
For front opening drum roasters the probe should be placed
somewhere in the lower quadrant of the uptake side of the drum. For clockwise rotating drums this would be
the lower left, for counterclockwise rotating drums the lower right. If you need to roast smaller batches then the
lower you should place the probe in the relevant quadrant. The most important thing to understand about
positioning a probe for accuracy is immersion, and immersion depth.
Immersion Depth
When attempting to place a probe for reading bean
temperature it is important to understand where on the probe (or more
accurately how much of the probe) the temperature measurement is actually
occurring. Temperature measuring occurs
at the end of the probe back and how much of the end of the probe is determined
by the diameter of the probe. You want
the probe to be immersed in the coffee to a length of 10 times the diameter of
the probe. So, if you have a probe
that is ¼” in diameter it should be surrounded by coffee for the last 2 ½” from
the tip back. Likewise, a 1/8” diameter
probe needs a 1 ¼” immersion depth.
These depths can be accomplished bending the probe if there is not
enough bean mass depth horizontally (see drawing).
Mounting the probe
The mistake most often made when mounting a probe through
the faceplate is not using the correct mounting hardware, or not using any
hardware at all. In picture 3 there are
2 Type J thermocouples both with the correct mounting hardware. This is a threaded compression fitting, one
end is threaded into the faceplate and the nut, with the compression barrel is
screwed into the fitting. The barrel
tightens against the probe, holding the probe in position. If a compression fitting is not used, then
the force of the coffee pushing against the probe can spin the probe out of
position, possibly damaging the probe.
Roasters often will use tape or some type of adhesive to hold a probe in
place, this is not recommended as it will loosen and can damage or even destroy
the probe.
When positioning the probe, before bending, and or
tightening it is important to ensure that you are not contacting either the
faceplate, the drum wall (this will throw off your measurements) or the fins
and supports for the drum wall (this can destroy the probe).
Bending the probe
It is often the case, as in picture 2 that a bean probe will
need to be bent in order to get an accurate and consistent bean temperature
reading. The probe in picture 2 is an
RTD, RTDs are easier to bend and can be bent using the thumb as a rest, and
bending over the pad of the thumb. It is
more difficult however to bend a Type J thermocouple and care must be taken not
to crimp or damage the interior of the probe.
It is best when bending a J thermocouple to use a mandrel (a pulley or
rod will suffice) that is at least 21/2 times the diameter of the probe. Do not attempt to bend a thermocouple without
using something to bend around. With
both probes you need to take care not to crimp the probe when bending for a 90
degree angle.
More Accurate Tools,
Safer Operation and Better Coffee
Regardless of which type probe you chose, or your reason for
choosing it, using a bean probe to help you monitor your coffee during roasting
will be one of the best business decisions you will ever make.
